Variable names can be arbitrarily long. They can contain both letters and
digits, but they have to begin with a letter or an underscore. Although it is
legal to use uppercase letters, by convention we don’t. If you do, remember
that case matters. Bruce and bruce are different variables.

Caution

Variable names can never contain spaces.

The underscore character ( _) can also appear in a name. It is often used in
names with multiple words, such as my_name or price_of_tea_in_china.
There are some situations in which names beginning with an underscore have
special meaning, so a safe rule for beginners is to start all names with a
letter.

If you give a variable an illegal name, you get a syntax error. In the example below, each
of the variable names is illegal.

76trombones is illegal because it does not begin with a letter. more$
is illegal because it contains an illegal character, the dollar sign. But
what’s wrong with class?

It turns out that class is one of the Python keywords. Keywords define
the language’s syntax rules and structure, and they cannot be used as variable
names.
Python has thirty-something keywords (and every now and again improvements to
Python introduce or eliminate one or two):

and

as

assert

break

class

continue

def

del

elif

else

except

exec

finally

for

from

global

if

import

in

is

lambda

nonlocal

not

or

pass

raise

return

try

while

with

yield

True

False

None

You might want to keep this list handy. If the interpreter complains about one
of your variable names and you don’t know why, see if it is on this list.

Programmers generally choose names for their variables that are meaningful to
the human readers of the program — they help the programmer document, or
remember, what the variable is used for.

Caution

Beginners sometimes confuse “meaningful to the human readers” with
“meaningful to the computer”. So they’ll wrongly think that because
they’ve called some variable average or pi, it will somehow
automagically calculate an average, or automagically associate the variable
pi with the value 3.14159. No! The computer doesn’t attach semantic
meaning to your variable names.

So you’ll find some instructors who deliberately don’t choose meaningful
names when they teach beginners — not because they don’t think it is a
good habit, but because they’re trying to reinforce the message that you,
the programmer, have to write some program code to calculate the average,
or you must write an assignment statement to give a variable the value you
want it to have.

Check your understanding

data-5-1: True or False: the following is a legal variable name in Python: A_good_grade_is_A+

(A) True

- The + character is not allowed in variable names.

(B) False

- The + character is not allowed in variable names (everything else in this name is fine).